England centre-back Maguire joined United in an £80million deal last August and succeeds Ashley Young, who is on the verge of a move to Inter.
United manager Ole Gunnar Solskjaer confirmed the news ahead of Monday's (AEDT) Premier League trip to Liverpool.
"Harry has been wearing the captain's armband and he will keep wearing it," Solskjaer told a news conference.
"He's been a leader in the group and I've been impressed by his leadership skills.
"He's come in and everything about him tells me he's a leader. He's been leading this young group.
"Harry will keep on wearing the armband now."